How to Size Solar Panel Systems


Solar panel systems are not a one-size-fits-all energy solution. The size system needed for a home varies depending on the residents' energy usage and the energy efficiency of the home, so it's possible that two homes with the same square footage can require different sized systems to meet their needs. Determining the appropriate size solar panel system your home needs only takes a few minutes of information gathering and a bit of simple math.

Things You'll Need

  • Last 12 months of electric bills
  • Calculator
  • Gather together your last 12 months of electric bills. If you don't have copies of your bills handy, your electric service provider can provide duplicates or you can look them up on your utility company's Web site if they provides online account management.

  • Note the kilowatts hours used on each bill. Your kilowatt hours used will generally be in the hundreds. Normally, the numbers for summer and winter months will be greater.

    For example:
    July's kilowatt hours used may be 469, but October's 287.

  • Calculate the average monthly kilowatt hours used by adding the 12 figures together, then dividing by 12.

    For example:
    469+489+374+222+252+284+332+356+283+287+346+469/12 = 346.916.
    We'll round up to 347 monthly kilowatt hours.

  • Calculate the kilowatt size of the solar system needed by dividing the average monthly kilowatt hours by the average hours of monthly sun. Use the following data for the average hours of monthly sun:
    150 in coastal areas,
    155 in mountain areas, and
    160 in sunny areas.

    For example:
    347/155 = 2.238
    We'll round down to 2.2 kW. The home in this example requires a 2.2 kW solar system.

  • Determine the number of solar panels needed for the system by multiplying the kilowatt system size by the number of solar panels needed to generate one kilowatt of electricity. Generating one kilowatt of energy requires 80 to 100 solar panels. Use the mid-point of the range, 90, for this calculation.

    For example:
    2.2 * 90 = 198
    The home in this example would need 198 solar panels installed as a part of its solar energy system to generate enough power to fulfill the average kilowatt hour usage of its residents.

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